Corneal refractive surgery, done for the purpose of reducing or eliminating the visual dependence on eyeglasses and contact lenses, started in 1978 in the U.S. with the first radial keratotomy procedures by Bores, who learned these procedures from Fyodorov of Moscow, Russia. Both Bores and Fyodorov taught radial keratotomy to many U.S. eye surgeons.
With the increasing use of corneal refractive surgery in the 1980s, a few surgeons added keratomileusis, a procedure originated by Barraquer of Bogota, Colombia, to their surgical armamentarium. Keratomileusis involves removing a contact lens shaped piece of the front of the human cornea, changing its curvature by freezing and lathing the inside of the removed tissue using a device called a cryolathe, and then suturing the corneal cap back onto the eye. Cryolathe keratomileusis was successful in eliminating eyeglass dependency in cases too extreme for the use of radial keratotomy but the freezing of the tissue was associated with a prolonged healing time.
During the late 1980s and early 1990s, Ruiz modified keratomileusis to eliminate the freezing step. The microkeratome was first used to cut a cap of tissue off the cornea as in the cryolathe keratomileusis. Then instead of modifying the resected corneal cap, a second incision of a small amount of tissue from the remaining eye tissue was used to produce optical correction. The cap was then replaced. Healing of the exterior of the eye was faster and more comfortable than in cryolathe keratomileusis. In the mid 1980s and again in the early 1990s, Ruiz modified the first step of his procedure by producing a flap with a nasal hinge rather than a cap. Ruiz's procedure was named automated lamellar keratoplasty. Although the healing was more rapid than cryolathe keratomileusis, the use of the second microkeratome to remove a very thin silver to change eye optics was flawed. It was difficult to produce second cuts to produce accurate enough optics and there were problems with contamination of outer covering cells being deposited under the repositioned corneal cap or flap.
By the early to mid 1990s, the excimer laser, a device initially to cut electronic circuit boards, had been modified by Trokel and others to remove the tissue directly from the eye to modify the eye curvature. At first the excimer laser was developed to make radial keratotomy incisions, but this failed. Then, the excimer was used to directly modify the corneal surface in a procedure called photorefractive keratoplasty or PRK. This procedure is used to the present day but is characterized by pain during early healing and a three-month period of full return of vision.
The excimer laser brought the ability to more precisely determine the center of the optical cut, but there was and is to this day, some debate as to where is the proper place to center the optical cut with the excimer laser. Some surgeons argue for placement over the center of the pupil and some for placement over the true optical center. This debate leads some surgeons to compromise and place the center of the optical cut somewhere between the center of the pupil and the true optical center of the eye. Certainly some patients have had excimer laser treatments where the optical center of treatment has been placed incorrectly.
Barraquer in 1984 proposed the “laser lathe” to replace the cryolathe step on the resected cap in cryolathe keratomileusis but he did not produce an instrument or do this surgery. Peyman proposed doing a microkeratome resection of the cornea and then placing the excimer ablation directly on the cornea beneath the resected tissue and then replacing the resected cap. He did not do this surgery, but outside the U.S., Burratto performed the first such procedure in 1990. Maloney, Epstein, and Brint separately performed the first intrastromal laser keratomileusis procedures in the United States in 1991. The name Lasik for Laser In Situ Keratomileusis (or Laser Intrastromal Keratomileusis) was given to the procedure.
In the present day, LASIK is performed by first using a microkeratome to produce a nasally hinged flap (or occasionally an unhinged cap) and then applying shaping excimer laser treatment to the eye tissue remaining after the resected tissue is moved out of the way. LASIK resembles Barraquer's keratomileusis and Ruiz's automated lamellar keratoplasty, but the excimer laser produces the tissue removal to change optics.
The Optical Centers, The Cornea and Vision
In all the corneal refractive surgeries mentioned, the treatment of myopia is the flattening of the central part of the human cornea. For each eye there is an optical center of the cornea. The optical center of the cornea is the projection of the visual axis on the cornea. The visual axis is that straight line that goes between the point of regard (such as a distant star) and the person's fovea. The optical center does not necessarily correspond to the geometric center of the cornea and is not necessarily in line with the center of the pupil.
Corneal topographic mapping is customary in the preoperative examination of a patient for refractive surgery, in particular for LASIK. Corneal topographic mapping is a common examination after corneal refractive surgery, especially in cases where vision is not satisfactory to the patient. Modern corneal topographic instruments can produce maps both of the curvature at different points on the cornea but also a map of the elevation of the cornea with respect to that of a best fitting perfect sphere.
After LASIK treatment, subtraction maps can be taken to determine where the excimer treatment actually occurred with respect to the true optical center of the cornea. When LASIK treatment is directed at the optical center of the cornea, then corneal topography elevation subtraction mapping shows a circle of color change corresponding to an elevation change which is centered on the elevation graph. Many surgeons have observed that when PRK or LASIK is performed where the laser-induced changes of the cornea do not correspond with the optical center of the cornea, there can be a blurring of vision or other various aberrations of vision. In other words, “off-center” ablations are associated with blurring of vision. The blurring of vision due to an irregular shape of the cornea is called “irregular astigmatism” and often cannot be corrected with spectacles and may only be correctable with hard contact lenses. Since people elect to have corneal refractive surgery because of the inability or dislike of wearing contact lenses, irregular astigmatism can be visually disabling.
Why Excimer Ablations Can Be Off-Center
Excimer laser ablations can be off center for various reasons: If the excimer laser contains a system to identify the edge of the pupil as the anchor for the position of the laser beam, then there are problems. There are problems because the center of the pupil is not necessarily in the center of the line of sight. Even if the center of the pupil were coincident with the line of sight, there can be problems. Since the pupil is located in a plane behind the plane of the cornea, it is possible for the optical center of the cornea to move out of the line of sight during a laser treatment if the eye is turned. If the laser beam is aligned with the pupil and not the cornea, then the treatment can smear over the cornea and produce an irregular result. Such turning of the cornea could easily happen. The patient cannot be relied upon to move the eye into the proper position. The patient may have poor vision without glasses. During a LASIK procedure, the cornea has been cut and so the patient's vision at the moment of excimer laser treatment is drastically reduced. Also, the patient is nervous and may not follow directions at the dramatic moment that the laser treatment is applied.
It is an object of the present invention to provide a method and apparatus for moving the optical center of an already accomplished excimer ablation to become on center.
Another option of the present invention is to provide a corrective laser ablation which is relatively simple in operation.
Other objects and advantages of the present invention will become apparent as the description proceeds.